U.S. patent number 6,012,782 [Application Number 08/790,340] was granted by the patent office on 2000-01-11 for apparatus for controlling differences in braking torque between left and right wheels of vehicle.
This patent grant is currently assigned to Denso Corporation, Toyota Jidosha Kabushiki Kaisha. Invention is credited to Takahiro Ikeuchi, Yousuke Takahira, Shinichi Yamada.
United States Patent |
6,012,782 |
Takahira , et al. |
January 11, 2000 |
Apparatus for controlling differences in braking torque between
left and right wheels of vehicle
Abstract
An apparatus for controlling a left-wheel and a right-wheel
braking torque applied to a left and a right wheel of a motor
vehicle running on a road surface, respectively, when the vehicle
is braked, including a torque changing device which changes each of
the left-wheel and right-wheel braking torques, and a controller
which controls, in an anti-lock control mode thereof, the torque
changing device to prevent each of the left and right wheels from
being locked on the road surface, the controller including
torque-difference control device for controlling the torque
changing device to change at least one of the left-wheel and
right-wheel braking torques such that a difference between the
left-wheel and right-wheel braking torques is smaller when a
deceleration of the vehicle is higher than a threshold value than
when the deceleration is not higher than the threshold value.
Inventors: |
Takahira; Yousuke (Toyota,
JP), Ikeuchi; Takahiro (Susono, JP),
Yamada; Shinichi (Anjo, JP) |
Assignee: |
Toyota Jidosha Kabushiki Kaisha
(Toyota-shi, JP)
Denso Corporation (Kariya-shi, JP)
|
Family
ID: |
11811527 |
Appl.
No.: |
08/790,340 |
Filed: |
January 27, 1997 |
Foreign Application Priority Data
|
|
|
|
|
Jan 29, 1996 [JP] |
|
|
8-12660 |
|
Current U.S.
Class: |
303/112; 303/169;
303/176 |
Current CPC
Class: |
B60T
8/348 (20130101); B60T 8/1764 (20130101); B60T
2201/16 (20130101) |
Current International
Class: |
B60T
8/17 (20060101); B60T 8/34 (20060101); B60T
8/1764 (20060101); B60T 008/58 () |
Field of
Search: |
;303/112,169,189,176 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Graham; Matthew C.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. An apparatus for controlling a left-wheel and a right-wheel
braking torque applied to a left and a right wheel of a motor
vehicle running on a road surface, respectively, when the vehicle
is braked, comprising:
a torque changing device which changes each of said left-wheel and
right-wheel braking torques; and
a controller which controls, in an anti-lock control mode thereof,
said torque changing device to prevent each of said left and right
wheels from being locked on the road surface,
said controller comprising torque-difference control means for
controlling said torque changing device to change at least one of
said left-wheel and right-wheel braking torques such that a
difference between the left-wheel and right-wheel braking torques
is smaller when a deceleration of the vehicle is higher than a
threshold value than when said deceleration is not higher than said
threshold value
wherein said controller further comprises:
normal control means for controlling said torque changing device to
change said at least one of said left-wheel and right-wheel braking
torques, so that said each of said left and right wheels is
prevented from being locked on the road surface; and
special control means for controlling said torque changing device
to change said at least one of said left-wheel and right-wheel
braking torques such that the difference between the left-wheel and
right-wheel braking torques when said special control means
controls the torque changing device is smaller than the difference
between the left-wheel and right-wheel braking torques when said
normal control means controls the torque changing device, so that
said each of said left and right wheels is prevented from being
locked on the road surface,
and wherein said torque-difference control means comprises
selecting means for, when said deceleration of the vehicle is
higher than said threshold value, selecting said special control
means for controlling said torque changing device, and inhibiting
said normal control means from controlling the torque changing
device and, when said deceleration is not higher than said
threshold value, selecting said normal control means for
controlling the torque changing device, and inhibiting said special
control means from controlling the torque changing device.
2. An apparatus according to claim 1, wherein said normal control
means comprises independent control means for selecting one of a
plurality of torque control modes, based on a parameter relating to
a lock tendency of each one of said left and right wheels,
independent of the other wheel, and controlling, in the selected
torque control mode, said torque changing device to change a
corresponding one of the left-wheel and right-wheel braking
torques, independent of the other braking torque, said plurality of
torque control modes comprising a torque-increase preventing mode
in which the torque changing device prevents each one of said
left-wheel and right-wheel braking torques from being
increased,
and wherein said special control means comprises first control
means for, in a case where two expected torque control modes are
different from each other, and where one of said two expected
torque control modes is equal to said torque-increase preventing
mode, controlling said torque changing device such that a
corresponding one of said left-wheel and right-wheel braking
torques is changed in said torque-increase preventing mode, and
such that the other braking torque is changed in one of said
plurality of torque control modes in which the torque changing
device is permitted to change said other braking torque to a value
smaller than a value to which said other braking torque would be
changed by the torque changing device in the other expected torque
control mode, said two expected torque control modes being expected
to be selected as one of said plurality of torque control modes
with relation to said left and right wheels, respectively, on
condition that said independent control means is operated with
relation to said left and right wheels.
3. An apparatus according to claim 2, wherein said first control
means comprises operating means for operating said independent
control means to select said one torque control mode based on said
parameter relating to said lock tendency of said each of said left
and right wheels and, when said different ones of said plurality of
torque control modes are selected for said left and right wheels,
respectively, and when said one of the selected torque control
modes is said torque-increase preventing mode, controlling said
torque changing device to change, in said torque-increase
preventing mode, said corresponding one of said left-wheel and
right-wheel braking torques and change the other braking torque in
said one of said plurality of torque control modes in which the
torque changing device changes said other braking torque to said
value smaller than said value to which said other braking torque
would be changed by the torque changing device in the other
selected torque control mode.
4. An apparatus according to claim 2, wherein said torque-increase
preventing mode comprises a torque decreasing mode in which said
torque changing device is permitted to decrease each one of said
left-wheel and right-wheel braking torques.
5. An apparatus according to claim 2, wherein said special control
means comprises second control means for, in a case where said two
expected torque control modes are different from each other, and
where one of said two expected torque control modes is equal to
said torque-increase preventing mode, controlling said torque
changing device such that each of said left-wheel and right-wheel
braking torques is changed in said torque-increased preventing
mode.
6. An apparatus according to claim 5, wherein said selecting means
comprises means for, when said deceleration of the vehicle is
higher than a first threshold value in an initial period of the
operation of said controller in said anti-lock control mode,
selecting said second control means for controlling said torque
changing device and inhibiting each of said independent control
means and said first control means from operating the torque
changing device; when said deceleration is higher than said first
threshold value and is not higher than a second threshold value
higher than said first threshold value after said initial period,
selecting said independent control means for controlling said
torque changing device and inhibiting each of said first and second
control means from operating the torque changing device; and, when
said deceleration is higher than said second threshold value higher
than said first threshold value after said initial period,
selecting said first control means for controlling said torque
changing device and inhibiting each of said independent and second
control means from operating the torque changing device.
7. An apparatus according to claim 5, wherein said controller
further comprises means for terminating the operation of said
second control means when said second control means has continued
controlling said torque changing device for more than a first
reference time.
8. An apparatus according to claim 5, wherein said second control
means comprises operating means for operating said independent
control means to select said one torque control mode based on said
parameter relating to said lock tendency of said each of said left
and right wheels and, when said different ones of said plurality of
torque control modes are selected for said left and right wheels,
respectively, and when said one of the selected torque control
modes is said torque-increase preventing mode, controlling said
torque changing device to change, in said torque-increase
preventing mode, said each of said left-wheel and right-wheel
braking torques.
9. An apparatus according to claim 2, wherein said controller
further comprises means for terminating the operation of said first
control means when said deceleration of the vehicle has decreased
down to lower than said second threshold value.
10. An apparatus according to claim 2, wherein said controller
further comprises means for terminating the operation of said first
control means when said first control means has continued to
control said torque changing device for more than a second
reference time and initiating the operation of said independent
control means to change said other braking torque in said other
selected torque control mode.
11. An apparatus for controlling a left-wheel and a right-wheel
braking torque applied to a left and a right wheel of a motor
vehicle running on a road surface, respectively, when the vehicle
is braked, comprising:
a torque changing device which changes each of said left-wheel and
right-wheel braking torques; and
a controller which controls, in an anti-lock control mode thereof,
said torque changing device to prevent each of said left and right
wheels from being locked on the road surface,
said controller comprising control means for, (a) when different
ones of a plurality of torque control modes comprising a torque
decreasing mode in which said torque changing device is permitted
to decrease each one of said left-wheel and right-wheel braking
torques, are selected based on respective parameters relating to
respective lock tendencies of said left and right wheels,
independent of each other, and (b) when one of said different
torque control modes which is selected for a corresponding one of
the left and right wheels is said torque decreasing mode,
controlling the torque changing device to change a corresponding
one of said left-wheel and right-wheel braking torques in said
torque decreasing mode and change the other braking torque in one
of said plurality of torque control modes in which the torque
changing device is capable of changing said other braking torque to
a value smaller than a value to which said other braking torque
would be changed by the torque changing device in the other of said
different torque control modes selected for the other of the left
and right wheels.
12. An apparatus according to claim 11, wherein said controller
further comprises mode selecting means for selecting said one of
said plurality of torque control modes based on said parameter
relating to the lock tendency of said each one of said left and
right wheels, independent of the other wheel.
13. An apparatus for controlling a left-wheel and a right-wheel
braking torque applied to a left and a right wheel of a motor
vehicle, comprising:
a torque changing device which changes each of said left-wheel and
right-wheel braking torques; and
a controller which controls said torque changing device to prevent
each of said left and right wheels from being locked,
said controller including control means for, in a case where two
expected torque control modes are different from each other, and
where one of said two expected torque control modes is equal to
said torque-increase preventing mode, controlling said torque
changing device such that a corresponding one of said left-wheel
and right-wheel braking torques is changed to said torque-increase
preventing mode, and such that the other braking torque is changed
in one of said plurality of torque control modes in which the
torque changing device is permitted to change said other braking
torque to a value smaller than a value to which said other braking
torque would be changed by the torque changing device in the other
expected torque control mode, said two expected torque control
modes being expected to be selected as one of a plurality of torque
control modes with relation to said left and right wheels,
respectively, independently of each other, on the basis of at least
one parameter relating to a lock tendency of each of said left and
right wheels, said plurality of torque control modes including said
torque-increase preventing mode in which the torque changing device
prevents each one said left-wheel and right-wheel braking torques
from being increased.
14. An apparatus according to claim 13, wherein said controller
further comprises mode selecting means for selecting one of said
plurality of torque control modes with relation to each one of said
left and right wheels, independently of the other wheel, on the
basis of said at least one parameter with relation to said each one
of said left and right wheels.
15. An apparatus for controlling a left-wheel and a right-wheel
braking torque applied to a left and a right wheel of a motor
vehicle running on a road surface, respectively, when the vehicle
is braked, comprising:
a torque changing device which changes each of said left-wheel and
right-wheel braking torques; and
a controller which controls, in an anti-lock control mode thereof,
said torque changing device to prevent each of said left and right
wheels from being locked on the road surface,
said controller comprising torque-difference control means for
controlling said torque changing device to change at least one of
said left-wheel and right-wheel braking torques such that a
difference between the left-wheel and right-wheel braking torques
is smaller when a deceleration of the vehicle is higher than a
threshold value than when said deceleration is not higher than said
threshold value
wherein the motor vehicle includes a front left wheel, a front
right wheel, a rear left wheel, and a rear right wheel, and wherein
said left and right wheels of the vehicle comprise said front left
and right wheels.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an anti-lock control apparatus for
a motor vehicle and particularly to the art of controlling the
difference between respective braking torques applied to a right
and a left wheel of the vehicle.
2. Related Art Statement
It is widely practiced to provide a motor vehicle with an anti-lock
control apparatus for not only shortening the braking distance
needed to brake and stop the vehicle running on a road surface but
also improving the stability of the running direction of the
vehicle when the vehicle is braked. Generally, an anti-lock control
apparatus controls respective braking torques applied to a left and
a right wheel of a vehicle and thereby prevents each wheel from
being locked on a road surface.
An example of the anti-lock control apparatus is disclosed in
Japanese Patent Application laid open for inspection under
Publication No. 62(1987)-275870. The prior apparatus can perform
(a) a normal control in which respective braking torques applied to
two front (left and right) wheels of a four-wheel vehicle are
controlled, independent of each other, based on respective lock
tendencies of the two wheels, and (b) a low-select control in which
the respective braking torques applied to the two front wheels are
controlled in a common manner based on the higher one of the
respective lock tendencies of the two wheels. More specifically
described, if an anti-lock control is started when the running
speed of the vehicle is not lower than a reference value, the
low-select control is selected commonly for the two front wheels
and is performed for a predetermined initial period of the
anti-lock control and subsequently the normal control is selected
and performed for each of the two front wheels, independent of each
other.
The prior apparatus performs the low-select control commonly for
the two front wheels, when the above-indicated initiating
conditions are satisfied, irrespective of whether the deceleration
of the vehicle is high or low when the vehicle is braked. However,
the degree of necessity to perform the low-select control for the
front wheels changes depending upon the vehicle deceleration.
When a four-wheel vehicle is subjected to a deceleration, the
vehicle is subjected to a forward load movement, so that vertical
loads exerted to front wheels of the vehicle increase and vertical
loads exerted to rear wheels of the vehicle decrease. Consequently
lateral forces exerted to respective tires of the rear wheels
decrease. The amount of forward load movement increases as the
vehicle deceleration increases. The vehicle deceleration increases
as friction coefficient, .mu., of road surface on which the vehicle
is running increases. As shown in FIG. 6, when a difference is
produced between respective braking torques applied to the front
left and right wheels, the vehicle is subjected to a yawing moment,
M, due to the torque difference. The change of the running
direction of the vehicle due to the yawing moment M cannot be
avoided unless the lateral forces which can stand the yawing moment
M are exerted to the tires of the rear wheels.
However, when the vehicle deceleration becomes high because the
vehicle is strongly braked on a road surface having a high friction
coefficient .mu., the amount of forward load movement becomes
large, so that the vertical loads exerted to the rear wheels become
low and the lateral forces exerted to the rear wheels also become
low. In this case, if the above-identified prior apparatus selects
and performs the low-select control for decreasing the difference
between the respective braking torques applied to the front left
and right wheels, the yawing moment M due to the torque difference
becomes low. Consequently the stability of the running direction of
the vehicle is maintained in spite of the low lateral forces
exerted to the tires of the rear wheels. Thus, when the vehicle
deceleration is high, the necessity to maintain the stability of
the vehicle's running direction is high and the necessity to
perform the front-wheel low-select control is also high.
In contrast, when the vehicle deceleration remains low because the
vehicle is braked on a road surface having a low friction
coefficient .mu., the amount of forward load movement is small, so
that the vertical loads exerted to the rear wheels remains
sufficiently high and the lateral forces exerted to the rear wheels
also remains high. In this case, even if the above-identified prior
apparatus does not perform the low-select control for the front
wheels, the stability of the running direction of the vehicle is
maintained. Accordingly, there remains a possibility that the
respective braking torques applied to the front wheels be increased
for shortening the braking distance. Thus, when the vehicle
deceleration is low, the necessity to maintain the stability of the
vehicle's running direction is lower than when the vehicle
deceleration is high, therefore the necessity to perform the
front-wheel low-select control is also lower.
While the above discussion is made for the low-select control
performed for the front left and right wheels, the following
discussion is made for the low-select control performed for the
rear left and right wheels:
Generally, when the low-select control is performed for the rear
left and right wheels, the slip ratios of the rear wheels becomes
lower than when the normal control is performed, and the lateral
forces exerted to the tires of the rear wheels become high. In
contrast, when the normal control is performed for the rear wheels,
the slip ratios of the rear wheels becomes higher than when the
low-select control is performed, and the braking forces applied to
the rear wheels become high. Therefore, when the vehicle
deceleration is high because the vehicle is braked on a road
surface having a high friction coefficient .mu. and accordingly the
vertical loads exerted to the rear wheels become low, it is
advantageous for the prior apparatus to select and perform the
low-select control for the rear wheels, for maintaining the
stability of the running direction of the vehicle. In contrast,
when the vehicle deceleration is low because the vehicle is braked
on a road surface having a low friction coefficient .mu. and the
vertical loads exerted to the rear wheels remains sufficiently
high, it is advantageous for the prior apparatus to select and
perform the normal control for the rear wheels, for shortening the
braking distance.
Thus, irrespective of whichever the low-select control is performed
for, the front wheels or the rear wheels, it is advantageous for
the prior apparatus to select and perform the low-select control
when the vehicle deceleration is high, and not to select the
low-select control when the vehicle deceleration is low, for the
purpose of improving its vehicle-braking performance including
shortening the vehicle-braking distance and maintaining the
stability of vehicle's running direction.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an
anti-lock control apparatus which exhibits improved vehicle-braking
performance irrespective of whether vehicle's deceleration is high
or low.
According to a first aspect of the present invention, there is
provided an apparatus for controlling a left-wheel and a
right-wheel braking torque applied to a left and a right wheel of a
motor vehicle running on a road surface, respectively, when the
vehicle is braked, comprising a torque changing device which
changes each of the left-wheel and right-wheel braking torques, and
a controller which controls, in an anti-lock control mode thereof,
the torque changing device to prevent each of the left and right
wheels from being locked on the road surface, the controller
comprising torque-difference control means for controlling the
torque changing device to change at least one of the left-wheel and
right-wheel braking torques such that a difference between the
left-wheel and right-wheel braking torques is smaller when a
deceleration of the vehicle is higher than a threshold value than
when the deceleration is not higher than the threshold value.
In the anti-lock control apparatus in accordance with the first
aspect of the invention, the torque-difference control means
controls the torque changing device to change the left-wheel and/or
right-wheel braking torques such that the difference between the
left-wheel and right-wheel braking torques is smaller when the
vehicle deceleration is higher than a threshold value than when the
deceleration is not higher than the threshold value. Therefore, the
present apparatus contributes to not only maintaining the direction
stability of the vehicle when the vehicle deceleration is high but
also shortening the braking distance of the vehicle when the
vehicle deceleration is low. The left and right wheels of the
vehicle may be a front left and a front right wheel of a four-wheel
vehicle, a rear left and a front right wheel of a four-wheel
vehicle, or a front and a rear left wheel and a front and a rear
right wheel of a four-wheel vehicle. In the case where the vehicle
is a four-wheel vehicle having a diagonal-type two-way braking
system, the left and right wheels of the vehicle may be a front
left and a rear right wheel, and/or a front right and a rear left
wheel, of the four-wheel vehicle. A braking torque applied to a
wheel may be changed by changing the fluid pressure (working fluid
may be a liquid or a gas) of a brake cylinder which operates a
brake device for reducing the rotation of the wheel. Alternatively,
a braking torque may be changed by changing an electric current or
voltage applied to an electric motor which operates a brake device.
Otherwise, a braking torque may be changed by changing a
regenerative power which is produced, when the vehicle is braked,
in an electric motor which drives the left and right wheels upon
application of an electric power thereto. The vehicle deceleration
may be detected by an exclusive deceleration-detecting device such
as a G switch or a G sensor. Alternatively, the deceleration may be
obtained as the time derivative of the running speed of the vehicle
estimated based on the respective rotation speeds of two or more
wheels of the vehicle. Otherwise, the deceleration may be
indirectly determined based on one or more parameters of the
vehicle which relate to the deceleration; such as driver's force
used to operate the brake pedal, brake pressure, pitching angle of
vehicle's body, vertical load exerted to each wheel, etc.
According to a preferred feature of the first aspect of the
invention, the controller further comprises normal control means
for controlling the torque changing device to change the at least
one of the left-wheel and right-wheel braking torques, so that the
each of the left and right wheels is prevented from being locked on
the road surface, and special control means for controlling the
torque changing device to change the at least one of the left-wheel
and right-wheel braking torques such that the difference between
the left-wheel and right-wheel braking torques when the special
control means controls the torque changing device is smaller than
the difference between the left-wheel and right-wheel braking
torques when the normal control means controls the torque changing
device, so that the each of the left and right wheels is prevented
from being locked on the road surface, and the torque-difference
control means comprises selecting means for, when the deceleration
of the vehicle is higher than the threshold value, selecting the
special control means for controlling the torque changing device,
and inhibiting the normal control means from controlling the torque
changing device and, when the deceleration is not higher than the
threshold value, selecting the normal control means for controlling
the torque changing device, and inhibiting the special control
means from controlling the torque changing device. In the case
where vehicle deceleration is high and lateral forces exerted to
tires of rear wheels of a four-wheel vehicle are low, so that the
demand to maintain the direction stability of the vehicle is
strong, the selecting means inhibits the normal control which
permits the difference between the left-wheel and right-wheel
braking torques to be greater than that permitted by the special
control. On the other hand, in the case where the vehicle
deceleration is low and the lateral forces exerted to the tires of
the rear wheels are high, so that the demand to maintain the
direction stability of the vehicle is not stronger than when the
deceleration is high, the selecting means inhibits the special
control which limits the generation of the difference between the
left-wheel and right-wheel braking torques. The normal control may
be an independent control in which one of a plurality of torque
control modes is selected based on a lock tendency of each one of
the left and right wheels, independent of the other wheel, and the
braking torque applied to each wheel is controlled in the selected
torque control mode. The special control may be a common control in
which the left-wheel and right-wheel braking torques are controlled
in a common torque control mode. The common control may be the
previously-described low-select control. The low-select control may
be of a type in which one of left and right wheels which is
identified as a locking wheel earlier than the other wheel and is
used as a reference wheel for determining a characteristic of the
current torque control, continues to be used as the reference wheel
throughout a certain anti-lock control operation. Otherwise, the
low-select control may be of a type in which the normal control is
virtually performed to select appropriate torque control modes for
the left and right wheels, respectively, and, if the selected modes
are different from each other, one of the two modes which permits a
more torque increase than the other mode is changed to the same
mode as the other mode. As far as the present invention is
concerned, any torque increase is defined as a more torque increase
than no torque change or any torque decrease, and no torque change
is defined as a more torque increase than any torque decrease. In
addition, a greater torque increase is defined as a more torque
increase than a smaller torque increase. In the latter case, when
only one of the left and right wheels is found as a locking wheel
and an anti-lock control operation is initiated for the locking
wheel only, the other wheel is not subjected to any anti-lock
control during at least the initial period of the anti-lock control
operation. That is, during the initial period, no torque control
mode is selected for the other wheel. However, driver's intention
permits the braking torque applied to the other wheel to be
increased. Thus, it can be said that a torque-increase mode has
been selected for the other wheel. Therefore, if in the initial
period the low-select mode is performed, the braking torque applied
to the other wheel is controlled in the same control mode as that
selected for the locking wheel.
According to another feature of the first aspect of the invention,
the normal control means comprises independent control means for
selecting one of a plurality of torque control modes, based on a
parameter relating to a lock tendency of each one of the left and
right wheels, independent of the other wheel, and controlling, in
the selected torque control mode, the torque changing device to
change a corresponding one of the left-wheel and right-wheel
braking torques, independent of the other braking torque, the
plurality of torque control modes comprising a torque-increase
preventing mode in which the torque changing device prevents each
one of the left-wheel and right-wheel braking torques from being
increased, and the special control means comprises first control
means for virtually operating the independent control means to
select the one torque control mode based on the parameter relating
to the lock tendency of the each of the left and right wheels and,
when different ones of the plurality of torque control modes are
selected for the left and right wheels, respectively, and
simultaneously when one of the selected torque control modes is the
torque-increase preventing mode, controlling the torque changing
device to change, in the torque-increase preventing mode, a
corresponding one of the left-wheel and right-wheel braking torques
and change the other braking torque in one of the plurality of
torque control modes in which the torque changing device is
permitted to change the other braking torque to a value smaller
than a value to which the other braking torque would be changed by
the torque changing device in the other selected torque control
mode. As mentioned above, the special control may be the low-select
control, and the low-select control may be of the type in which the
normal control is virtually performed to select respective
appropriate control modes for the left and right wheels and, if the
selected two modes are different from each other, one of the two
modes which permits a more torque increase than the other mode is
changed to the same mode as the other mode. Accordingly, in the
case where in the normal control a torque increasing mode would be
selected as the torque control mode, in the special control the
torque increasing mode is changed to a torque holding mode or a
torque decreasing mode. Thus, the low-select control is one of
different types of special controls which has a high tendency to
decrease the braking force applied to each one of the left and
right wheels. Meanwhile, in the case where a torque decreasing and
a torque increasing mode are selected and used for a left and a
right wheel, or in the case where a torque decreasing and a torque
holding mode are selected and used for a left and a right wheel,
the difference between respective braking torques applied to the
two wheels may be increased as a result of the anti-lock control
under the selected modes. This difference may be increased also in
the case where a torque increasing and a torque holding mode are
selected and output for a left and a right wheel. In the third
case, however, if one of the left-wheel and right-wheel braking
torques is controlled in a torque decreasing mode in place of the
mode virtually selected therefor, the sum of respective braking
forces applied to the two wheels may be reduced excessively
largely.
Hence, according to another feature of the first aspect of the
invention, the torque-increase preventing mode may comprise a
torque decreasing mode in which the torque changing device is
permitted to decrease each one of the left-wheel and right-wheel
braking torques. According to this feature, one of the two selected
modes which permits a more torque increase as defined above than
the other mode is not changed to a different mode which permits a
less torque increase, in all cases in which the two modes are
different from each other. Only in the case where the two selected
modes includes a torque decreasing mode, the above one selected
mode is changed to the above different mode. Thus, this type of
special control has a lower tendency to reduce the sum of
respective braking forces applied to the left and right wheels,
than the low-select control, and contributes to shortening the
braking distance needed to brake and stop the vehicle. In the case
where the torque changing device is operable in each of a torque
increasing mode, a torque holding mode, and a torque decreasing
mode, the torque increasing mode virtually selected by the
independent control means may be changed to the torque decreasing
mode or the torque holding mode by the first control means. In
addition, in the case where the torque changing device is operable
in each of a quick torque increasing mode and a slow torque
increasing mode, the quick torque increasing mode virtually
selected by the independent control means may be changed to the
slow torque increasing mode by the first control means. In short,
in the special control, a torque control mode which is virtually or
provisionally selected by the independent control means may be
changed to a different torque control mode which permits a less
torque increase as defined above than that permitted by the
selected mode.
According to another feature of the first aspect of the invention,
the special control means comprises second control means for
virtually operating the independent control means to select the one
torque control mode based on the parameter relating to the lock
tendency of the each of the left and right wheels and, when
different ones of the plurality of torque control modes are
selected for the left and right wheels, respectively, and
simultaneously when one of the selected torque control modes is the
torque-increase preventing mode, controlling the torque changing
device to change, in the torque-increase preventing mode, each of
the left-wheel and right-wheel braking torques.
According to another feature of the first aspect of the invention,
the selecting means comprises means for, when the deceleration of
the vehicle is higher than a first threshold value as the threshold
value in an initial period of the operation of the controller in
the anti-lock control mode, selecting the second control means for
controlling the torque changing device and inhibiting each of the
independent control means and the first control means from
operating the torque changing device; when the deceleration is
higher than the first threshold value and is not higher than a
second threshold value higher than the first threshold value after
the initial period, selecting the independent control means for
controlling the torque changing device and inhibiting each of the
first and second control means from operating the torque changing
device; and, when the deceleration is higher than the second
threshold value higher than the first threshold value after the
initial period, selecting the first control means for controlling
the torque changing device and inhibiting each of the independent
and second control means from operating the torque changing
device.
According to another feature of the first aspect of the invention,
the controller further comprises means for terminating the
operation of the second control means when the second control means
has continued to control the torque changing device for more than a
first reference time.
According to another feature of the first aspect of the invention,
the controller further comprises means for terminating the
operation of the first control means when the deceleration of the
vehicle has decreased down to lower than the second threshold
value.
According to another feature of the first aspect of the invention,
the controller further comprises means for terminating the
operation of the first control means when the first control means
has continued controlling the torque changing device for more than
a second reference time and initiating the operation of the
independent control means to change the other braking torque in the
other selected torque control mode.
According to another feature of the first aspect of the invention,
the motor vehicle includes a front left wheel, a front right wheel,
a rear left wheel, and a rear right wheel, and wherein the left and
right wheels of the vehicle comprise the front left and right
wheels.
According to a second aspect of the present invention, there is
provided an apparatus for controlling a left-wheel and a
right-wheel braking torque applied to a left and a right wheel of a
motor vehicle running on a road surface, respectively, when the
vehicle is braked, comprising a torque changing device which
changes each of the left-wheel and right-wheel braking torques, and
a controller which controls, in an anti-lock control mode thereof,
the torque changing device to prevent each of the left and right
wheels from being locked on the road surface, the controller
comprising control means for, (a) when different ones of a
plurality of torque control modes comprising a torque decreasing
mode in which the torque changing device is permitted to decrease
each one of the left-wheel and right-wheel braking torques, are
selected based on respective parameters relating to respective lock
tendencies of the left and right wheels, independent of each other,
and (b) when one of the different torque control modes which is
selected for a corresponding one of the left and right wheels is
the torque decreasing mode, controlling the torque changing device
to change a corresponding one of the left-wheel and right-wheel
braking torques in the torque decreasing mode and change the other
braking torque in one of the plurality of torque control modes in
which the torque changing device is permitted to change the other
braking torque to a value smaller than a value to which the other
braking torque would be changed by the torque changing device in
the other of the different torque control modes selected for the
other of the left and right wheels.
In the anti-lock control apparatus in accordance with the second
aspect of the invention, one of the two selected modes which
permits a more torque increase as defined above than the other mode
is not changed to a different mode which permits a less torque
increase, in all cases in which the two modes are different from
each other. Only in the case where the two selected modes includes
a torque decreasing mode, the above one selected mode is changed to
the above different mode. Thus, this torque control enjoys a lower
tendency to reduce the sum of braking forces applied to the left
and right wheels, than the low-select control, and contributes to
shortening the vehicle braking distance.
According to a preferred feature of the second aspect of the
invention, the controller further comprises mode selecting means
for selecting the one of the plurality of torque control modes
based on the parameter relating to the lock tendency of the each
one of the left and right wheels, independent of the other
wheel.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and optional objects, features, and advantages of the
present invention will be better understood by reading the
following detailed description of the preferred embodiments of the
invention when considered in conjunction with accompanying
drawings, in which:
FIG. 1 is a diagrammatic view of an anti-lock braking system
including an anti-lock control apparatus to which the present
invention is applied;
FIG. 2 is a view for explaining a front-wheel low-select control
performed by a control unit of the braking system of FIG. 1;
FIG. 3 is a view for explaining a front-wheel mode-change control
carried out by the control unit of the braking system of FIG.
1;
FIG. 4 is a flow chart representing an anti-lock control routine
according to which the control unit of the braking system of FIG. 1
operates for selectively performing the front-wheel low-select
control, the front-wheel mode-change control, and a front-wheel
normal control;
FIG. 5 is a time chart showing an example of a time-wise change of
respective brake pressures applied to a front left (FL) wheel and a
front right (FR) wheel of a four-wheel vehicle under control of the
control unit of the braking system of FIG. 1 according to the
control routine represented by the flow chart of FIG. 4; and
FIG. 6 is a view for explaining a relationship among the difference
between respective braking forces applied to the front left and
right wheels, respective lateral forces exerted to a rear left
wheel and a rear right wheel of the vehicle, and a yawing moment
exerted to the vehicle.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, there will be described an anti-lock
braking system including an anti-lock control apparatus to which
the present invention is applied.
The present braking system is a diagonal two-way braking system for
a four-wheel motor vehicle. As shown in FIG. 1, the braking system
includes a master cylinder (M/C) 10 which is connected to four
brake cylinders 16 via a brake unit 12 and a proportioning ("P")
valve 14.
The master cylinder 10 is a tandem cylinder in which two
independent pressure chambers are provided in series. When a brake
pedal 20 functioning as a brake-system operating member is pressed
by a driver's foot, an identical fluid pressure corresponding to
the driver's pressing force applied to the pedal 20 is mechanically
produced in each of the two pressure chambers of the master
cylinder 10.
The brake unit 12 includes a first and a second main passage 24, 26
which are independent of each other. The first main passage 24
connects between one of the two pressure chambers of the master
cylinder 10 and the two brake cylinders 16 associated with a front
right wheel, FR, and a rear left wheel, RL. The second main passage
26 connects between the other pressure chamber of the master
cylinder 10 and the two brake cylinders 16 associated with a front
left wheel, FL, and a rear right wheel, RR.
The first main passage 24 includes a trunk passage 30 and two
branch passages 32 bifurcated from the trunk passage 30. The trunk
passage 30 is connected via a piping 34 to one pressure chamber of
the master cylinder 10. The two branch passages 32 are connected
via respective pipings 36, 38 to the respective brake cylinders 16
for the two wheels FR, RL.
A first solenoid valve 40 which is normally open is provided in
each of the branch passages 32, and normally permits the fluid
pressure of the master cylinder 10 to be transmitted to the brake
cylinders 16 for the wheels FR, RL. A return passage 44 which is
provided with a check valve 42 and which bypasses the solenoid
valve 40 is also provided in each of the branch passages 32. A
reservoir passage 46 is connected at one end thereof to each branch
passage 32 at a point intermediate between each solenoid valve 40
and a corresponding brake cylinder 16 and is connected at the other
end thereof to a reservoir 48. A second solenoid valve 50 which is
normally closed is provided in each of the reservoir passages 46,
and normally prevents the brake fluid from being discharged from a
corresponding brake cylinder 16 to the reservoir 48.
A pump passage 52 is connected at one end thereof to a joined
reservoir passage 46 and is connected at the other end thereof to
the master cylinder 10. A pump 54 and a damper 56 are provided in
the pump passage 52. The pump 54 pumps up the brake fluid from the
reservoir 48 and returns it to the master cylinder 10. The damper
56 damps the pulsation of brake fluid caused by the pump 54.
The second main passage 26 has the same construction as the
above-described construction of the first main passage 24. The same
reference numerals as used for the various elements associated with
the first main passage 24 are used to designate the corresponding
elements of the second main passage 26, and the description thereof
is omitted. However, the two pumps 54 provided for the two
brake-pressure ways 24, 26 are driven by a common electric motor
58.
The P (proportioning) valve 14 is provided in the two pipings 38
which connect between the brake unit 12 and the respective brake
cylinders 16 for the two rear wheels RL, RR. When the fluid
pressure of the master cylinder 10 is not higher than a threshold
value, the P valve 14 transmits the fluid pressure to each cylinder
16 without modifying it. On the other hand, when the fluid pressure
of the master cylinder 10 is higher than the threshold value, the P
valve 14 transmits decreased fluid pressures to the two cylinders
16, respectively. The P valve 14 includes two valves which can
decrease the fluid pressure independently of each other, and a
common housing in which the two valves are accommodated.
The respective solenoids of the first and second solenoid valves
40, 50 are connected to the control unit 60. The control unit 60 is
essentially provided by a computer 70 including a central
processing unit (CPU) 62, a read only memory (ROM) 64, and a random
access memory (RAM) 66. Four wheel-speed sensors 72 which measure
respective peripheral speeds of the four wheels FR, RL, RR, FL are
also connected to the control unit 60. The control unit 60 controls
the first and second solenoid valves 40, 50, based on detection
signals supplied from the four sensors 72. When the driver's foot
presses the brake pedal 20, the control unit 60 controls, via the
solenoid valves 40, 50, the brake pressure applied to each wheel
FR, RL, RR, FL, such that each wheel is effectively prevented from
being locked on a road surface on which the vehicle is running.
In the present embodiment, the first and second solenoid valves 40,
50, the wheel-speed sensors 72, and the control unit 60 cooperate
with one another to provide the anti-lock control apparatus. The
anti-lock control apparatus performs, as the normal control, an
independent control in which the control unit 60 controls, based on
a lock tendency of each of the four wheels FR, RL, RR, FL, the
brake pressure applied to that wheel, independent of the control of
the other brake pressures applied to the other wheels. However, if
at the beginning of each anti-lock control operation a deceleration
of the vehicle's body is not lower than a first reference value, A,
(e.g., 0.6 [g]), the control unit 60 performs, for the front left
and right wheels FL, FR, a low-select control as a first type of
special control, i.e., left-and-right-wheel torque-difference
limiting control. If the deceleration of the vehicle's body exceeds
a second reference value, B, (e.g., 0.8 [g]) higher than the first
reference value A, the control unit 60 performs, for the two front
wheels FL, FR, a mode-change control as a second type of special
control. For the rear wheels RL, RR, the control unit 60 always
performs the same low-select control as that performed for the
front wheels FL, FR, in each anti-lock control operation. However,
for the rear wheels, it is possible to perform the independent
control in place of the low-select control.
Next, there will be described an initiating and a terminating
condition, and details, of the front-wheel low-select control.
(1) Initiating Condition
As shown in FIG. 2, the low-select control is initiated when the
following three conditions are simultaneously satisfied: the first
condition is that the current fluid-pressure control period is the
initial pressure-decrease control period in the current anti-lock
control operation; the second condition is that the current running
speed of the vehicle's body is not lower than a reference value;
and the third condition is that the current deceleration of the
vehicle's body is not lower than the first reference value A.
In the present embodiment, each anti-lock control operation
includes repetitive unit pressure control periods each of which
includes a single pressure-decrease control period and a single
pressure-increase control period. In each unit pressure control
period, the pressure-decrease control period precedes the
pressure-increase control period. Thus, the above-indicated first
condition means that the current fluid-pressure control period is
the pressure-decrease control period of the first or initial unit
pressure control period, that is, the first or initial
pressure-decrease control period in the current anti-lock control
operation.
The above-indicated second condition is provided because no
reliable anti-lock control operation can be performed before it is
judged that the wheel-speed sensors 72 are supplying accurate
signals to the control unit 60. For making this judgment, it is
required that the running speed of the vehicle's body be higher
than a certain reference value. This reference value is, e.g., 30
km/h. Since the second condition is not essential according to the
principle of the present invention, it may be omitted.
(2) Details
In the low-select control, the respective brake pressures applied
to the front left and right wheels FL, FR are controlled in a
common mode determined based on a current lock tendency of one of
the two front wheels FL, FR which is higher than that of the other
wheel. More specifically described, first, the control unit 60
selects, for each of the two front wheels FL, FR, an appropriate
one of a pressure-increase ("PI") mode, a pressure-hold ("PH")
mode, and a pressure-decrease ("PD") mode which are employed in the
above-described independent control, based on the current lock
tendency of each of the front wheels FL, FR, assuming that the
current anti-lock control operation is under the independent
control. Then, the control units 60 compares the two control modes
selected for the two front wheels FL, FR, with each other, and
selects one of the two modes which has a lower pressure-change
level or degree than that of the other mode. As far as the present
invention is concerned, the pressure-change level of the PH mode is
defined as being lower than that of the PI mode and higher than
that of the PD mode. For example, if in the assumed independent
control the PI mode is selected for one of the front wheels FL, FR
and the PD mode is selected for the other, then, the control unit
60 selects, in the low-select control, the PD mode commonly for the
two wheels FL, FR, so that an identical braking torque is applied
to each of the front wheels FL, FR and the difference between the
respective braking forces applied to the front wheels FL, FR is
minimized.
In the present embodiment, the PI mode includes a quick or
"continuous" PI mode in which each first solenoid valve 40 is
continuously held in a PI state thereof, and a slow or "pulse" PI
mode (i.e., intermittent PI mode) in which each valve 40 is
alternately switched to the PI state and a PD state thereof in
response to pulse signals supplied from the control unit 60. As far
as the present embodiment is concerned, the continuous and pulse PI
modes each as a PI mode are defined as having an equal
pressure-change level. However, it is possible to define the
continuous and pulse increase modes such that the continuous PI
mode has a pressure-change level higher than that of the pulse PI
mode.
When the pressing of the driver's foot on the brake pedal 20 is
started, an initial period may be observed in which only one of the
front wheels FL, FR exhibits an excessively high lock tendency and
an anti-lock control operation is started for that one front wheel
only. In this initial period, no control mode is selected for the
other front wheel. However, since the driver's intention permits an
increased braking torque to be applied to the other wheel, it can
be said that the PI mode has been selected for the other wheel.
Therefore, when in the initial period the low-select control is
performed for the two front wheels FL, FR, the same control mode as
that requested for one of the two wheels is selected for the other
wheel.
(3) Terminating Condition
As shown in FIG. 2, the low-select control is terminated when
either one of the following two conditions is satisfied: the first
condition is that in the independent control the PI mode would be
selected for each of the two front wheels FL, FR; and the second
condition is that the current low-select control has been continued
for more than a reference time, T.sub.1 (sec).
Next, there will be described an initiating and a terminating
condition, and details, of the front-wheel mode-change control.
(1) Initiating Condition
As shown in FIG. 3, the mode-change control is initiated when both
of the following two conditions are satisfied; the first condition
is that an anti-lock control operation is being performed for one
or both of the two front wheels FL, FR; and the second condition is
that the vehicle deceleration is not lower than the second
reference value B.
(2) Details
Assuming that the independent control has been selected, the
control unit 60 initially determines a control mode requested for
each of the two wheels FL, FR. If the PD mode is requested for only
one of the two wheels, the control unit 60 does not change the PD
mode and change the control mode initially determined for the other
wheel, to a lower pressure-change-level mode in which the brake
pressure will be lower than will be in the initially determined
mode.
The details of the mode-change control are summarized in a table
shown in FIG. 3. In the table, the "object" wheel is one of the two
wheels which is being focused, and the "opposite" wheel is the
other of the two wheels. The PD, PH, and PI modes are modes which
would be selected or requested in the independent control.
In the case where in the independent control the PD mode would be
requested for the opposite wheel, the PD mode which in the
independent control would be requested for the object wheel is not
changed and is finally adopted for the object wheel; and in the
case where the PH or PI mode which in the independent control would
be requested for the object wheel is changed to a lower
pressure-change-level mode, i.e., PD mode, which is finally adopted
for the object wheel. In the case where in the independent control
the PH or PI mode would be requested for the opposite wheel, any of
the PD, PH, and PI modes which in the independent control would be
requested for the object wheel is not changed and is finally
adopted for the object wheel.
The above description relating to the changing of the mode
initially requested for the object wheel can apply to the opposite
wheel, as shown in the table of FIG. 3.
(3) Terminating Condition
As shown in FIG. 3, the mode-change control is terminated when
either one of the following two conditions is satisfied; the first
condition is that it is after an anti-lock control operation is
terminated and before another anti-lock control operation is
initiated; and the second condition is that the vehicle
deceleration is lower than the second reference value B.
If the mode-change control is continued till the first or second
condition is satisfied, then it may be possible that the brake
pressure being applied to one wheel under the anti-lock control in
the lower pressure-change-level mode which has been changed in the
mode-change control from the higher pressure-change-level mode
which would be selected in the independent control, may be
decreased excessively largely. On the other hand, when the current
mode-change control is terminated and simultaneously the
independent control is initiated, the higher pressure-change-level
mode is selected and output. Since the higher pressure-change-level
mode cannot be the PD mode, no further decreasing of the brake
pressure being applied to the wheel in question is continued.
Hence, in the present embodiment, the mode-change control is
carried out in such a way that the lower pressure-change-level mode
which has been changed from the higher pressure-change-level mode
which would be selected in the independent control, is continued
for a short reference time, T.sub.2, and subsequently is changed
back to the higher pressure-change-level mode. Thus, the
mode-change control is not continued for more than the reference
time T.sub.2.
The above-described anti-lock control is carried out according to
various routines pre-stored in the ROM 64. Those routines include a
wheel-speed calculating routine, a vehicle-speed estimating
routine, an anti-lock-control-initiation judging routine, and an
anti-lock-control performing routine.
The wheel-speed calculating routine is carried out at a regular
interval of time, so as to calculate the speed of rotation of each
wheel 16 based on the signal supplied from a corresponding
wheel-speed sensor 72.
The vehicle-speed estimating routine is carried out at a regular
interval of time, so as to estimate the running speed of the
vehicle's body based-on the respective rotation speeds of the four
wheels FL, FR, RL, RR.
The anti-lock-control-initiation judging routine is carried out for
each of the four wheels FL, FR, RL, RR, so as to judge whether each
wheel exhibits an excessively large lock tendency, based on a
relationship between the calculated rotation speed of each wheel
and the estimated running speed of the vehicle. However, since in
the present embodiment the principle of the present invention is
applied to the anti-lock control of the front wheels FL, FR only,
the following description relates to only the front wheels and the
description relating to the rear wheels is omitted. If a positive
judgment is made for either one of the front wheels, the control
unit 60 carries out the anti-lock-control performing routine
represented by the flow chart of FIG. 4.
The anti-lock-control performing routine is carried out for each of
the front wheels FL, FR, so as to select an appropriate one of the
independent control, the low-select control, and the mode-change
control, based on given conditions.
The anti-lock control performing routine of FIG. 4 is carried out
in repetitive control cycles. In each control cycle in accordance
with the routine of FIG. 4, first, at Step S1, the CPU 62 of the
control unit 60 calculates the deceleration of vehicle's body based
on the difference between two running speeds of vehicle's body
estimated in two successive (current and prior) control cycles in
accordance with the vehicle-speed estimating routine. Step S1 is
followed by Step S2 to judge whether one or both of the front
wheels FL, FR is now under the anti-lock control in accordance with
the routine of FIG. 4. If a negative judgment is made at Step S2,
the current control cycle in accordance with the routine of FIG. 4
is terminated. On the other hand, if a positive judgment is made at
Step S2, the control of the CPU 62 goes to Step S3 and the
following steps.
At Step S3, the CPU 62 judges whether the vehicle deceleration
calculated at Step S1 is not lower than the first reference value
A. Assuming that the vehicle deceleration is higher than the
reference value A because the driver has strongly pressed the brake
pedal 20 on a road surface having a high friction coefficient,
.mu., a positive judgment is made at Step S3, and the control of
the CPU 62 goes to Step S4. At Step S4, the CPU 62 judges whether
either one of the front wheels FL, FR is now in the initial
pressure-decrease control period of the current anti-lock control
operation. If a positive judgment is made at Step S4, the control
of the CPU 62 goes to Step S5 to select the low-select control for
the two wheels FL, FR, and the current control cycle is terminated.
During the initial period of the anti-lock control operation, the
other wheel does not exhibit an excessively large lock tendency.
However, the decreasing of the brake pressure being applied to the
other wheel may be started to comply with the PD control requested
for the one wheel. Thus, the respective brake pressures being
applied to the two front wheels FL, FR are made equal to each
other, so that the vehicle is prevented from being subjected to a
yawing moment which would otherwise change the running direction of
the vehicle. Consequently the running stability of the vehicle that
tends to be lowered during the initial period in the anti-lock
control operation, is not lowered. Thus, the braking performance of
the vehicle is improved.
The low-select control is terminated when the first or second
terminating condition therefor is satisfied. Assuming that the
routine of FIG. 4 is started again after the initial PD control
period and that the current vehicle deceleration is not lower than
the first reference value A and is lower than the second reference
value B, a positive judgment is made at each of Steps S2 and S3 and
a negative judgment is made at each of Steps S4 and S6, so that the
control of the CPU 62 goes to Step S8 to select the independent
control for each of the two wheels FL, FR. Thus, one control cycle
in accordance with the routine of FIG. 4 is ended. Consequently the
brake pressures being applied to the two wheels FL, FR are
controlled or changed, as shown in FIG. 5, while those pressures
are permitted to have a great difference.
Assuming that subsequently the vehicle deceleration increases up to
a value higher than the second reference value B, a positive
judgment is made at each of Steps S2 and S3, a negative judgment is
made at each of Step S4, and a positive judgment is made at Step
S6, so that the control of the CPU 62 goes to Step S7 to select the
mode-change control for the two wheels FL, FR. In the mode-change
control, when the PD mode is requested for one wheel, a higher
pressure-change-level mode which would be requested for the other
wheel in the independent control is changed to a lower
pressure-change-level mode, e.g., PD mode. Thus, the difference
between the respective brake pressures being applied to the two
wheels FL, FR is prevented from being increased.
Assuming that in the current anti-lock control operation the
vehicle deceleration remains lower than the first reference value
A, a negative judgment is made at Step S3, and the control of the
CPU 62 goes to Step S8 to select the independent control for the
two wheels FL, FR.
Meanwhile, assuming that the current pressure control is now in the
initial PD control period and that the vehicle deceleration is
lower than the first reference value A because the driver has
pressed the brake pedal 20 on a road surface having a low friction
coefficient .mu., a positive judgment is made at Step S2 and a
negative judgment is made at Step S3, and the control of the CPU 62
goes to Step S8 to select the independent control. Thus, one
control cycle in accordance with the routine of FIG. 4 is
ended.
It emerges from the foregoing description that in the present
embodiment, in the case where the vehicle deceleration is lower
than the first reference value A, neither of the low-select control
and the mode-change control each as a left-and-right-wheel
torque-difference limiting control is carried out for the front
wheels FL, FR, but the independent control is carried out.
Therefore, when the driver quickly presses the brake pedal 20 on
the road surface whose friction coefficient .mu. is low, the sum of
respective braking forces applied to the two wheels FL, FR is
maximized and accordingly the braking distance is easily
shortened.
The low-select control is a left-and-right-wheel torque-difference
limiting control in which the generation of difference between the
braking pressures applied to the left and right front wheels FL, FR
is strictly limited for positively preventing the change of running
direction of the vehicle, whereas the mode-change control is
another left-and-right-wheel torque-difference limiting control in
which the generation of difference between the braking pressures
applied to the two front wheels FL, FR is less strictly limited for
less positively preventing the change of running direction of the
vehicle.
In the case where the driver strongly presses the brake pedal 20 on
the road surface with the high friction coefficient .mu., if the
low-select control is started before the vehicle deceleration
increases up to a sufficiently high value, for the purpose of
preventing early the change of running direction of the vehicle due
to the production of difference between the braking pressures
applied to the left and right front wheels FL, FR, then the
necessity to continue the low-select control for positively
preventing the change of running direction of the vehicle is
significantly reduced after the vehicle deceleration has increased
up to the sufficiently high value, that is, the mode-change control
which contributes to increasing the sum of respective braking
forces applied to the two wheels FL, FR suffices for achieving the
aim of preventing the change of running direction of the vehicle.
Thus, in this case, the low-select control is started before the
vehicle deceleration has increased up to the sufficiently high
value.
In the present embodiment, both the low-select control and the
mode-change control are employed each as the left-and-right-wheel
torque-difference limiting control. In the case where the driver
strongly presses the brake pedal 20 on the road surface with the
high friction coefficient .mu., if the vehicle deceleration is not
lower than the first reference value A and is lower than the second
reference value B higher than the first value A, the low-select
control is selected, and if the vehicle deceleration is not lower
than the second reference value B, the mode-change control is
selected. Thus, both the prevention of change of running direction
of the vehicle's body and the shortening of braking distance of the
vehicle's body are simultaneously achieved with satisfaction.
As is apparent from the foregoing description, the routine of FIG.
4 and a portion of the control unit 60 for carrying out the routine
cooperate with each other to provide a torque-difference control
means.
While the present invention has been described in its preferred
embodiment, the present invention may otherwise be embodied.
For example, in the illustrated embodiment, the
left-and-right-wheel torque-difference limiting rule in which the
mode-change control as the left-and-right-wheel torque-difference
limiting control is used to control the braking torques applied to
the wheels of the vehicle is employed together with the technique
of selecting, based on the vehicle deceleration, an appropriate one
of the torque-difference limiting rule and the normal rule in which
the independent control is used for the same purpose. However, it
is possible to employ the mode-change control independent of that
technique. For example, the mode-change control may be employed
with the technique of fixing a braking-torque control rule
independent of the vehicle deceleration; it may be employed as a
torque control which is performed before the low-select control is
performed; or it may be employed in an anti-lock control in which
no low-select control is employed.
It is to be understood that the present invention may be embodied
with other changes, improvements, and modifications that may occur
to those skilled in the art without departing from the scope and
spirit of the invention defined in the appended claims.
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